Shear Lag Transfer Length
Shear Lag Transfer Length
(OP)
In short, over what distance of a tension member does shear lag need to be considered?
If I have a 20' pin connected member, with a 2' connection at each end, my intuition is that the shear lag effect of that connection (in this case, a knife plate gusset to an HSS) probably only applies to the end 4-5' of the member before tensile stresses are distributed to the full cross section. After this 4-5', it would seem rational to use the yield on gross capacity, especially for combined stress checks.
That said, I can't find any provision of the codes that says this. Am I totally off base?
If I have a 20' pin connected member, with a 2' connection at each end, my intuition is that the shear lag effect of that connection (in this case, a knife plate gusset to an HSS) probably only applies to the end 4-5' of the member before tensile stresses are distributed to the full cross section. After this 4-5', it would seem rational to use the yield on gross capacity, especially for combined stress checks.
That said, I can't find any provision of the codes that says this. Am I totally off base?






RE: Shear Lag Transfer Length
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Shear Lag Transfer Length
RE: Shear Lag Transfer Length
1) Go to the point "l" away from the member that represents the end of the connection proper.
2) Trace your way from the connected element at "l" around the section to the point furthest away, travelling at a 45 degree angle.
3) Assume that the point that you end up at represents the location of U=1.0.
Once you're outside of the connection and into member propoer, load spread should occur very rapidly. I've always assumed that web yielding provisions imply a distribution angle of about 2.5:1. By that measure, 45 degrees should be pretty conservative.
If you want to be more aggressive, you could modify the procedure above such that the starting point occurred a distance across the section that assumes that all of the effective net area is located as close as possible to the connected element.
What are you trying to accomplish here anyhow? I still contend that your D3 check is all that is required for shear lag.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Shear Lag Transfer Length
Of course, this may be unconservative for a connection that is infinitely long, so use the most conservative answer when compare's to KootK's response.
The code is meant to express the importance of Saint Venant's principle and shear lag. If you have a special case where this does not reasonably apply (because you understand the code's intent), then the code restriction does not apply. There's always a code provision for best judgement.
EDIT: For combined stress checks: Combined stress checks can be segmented however you choose. They're based on Ma/Mr and Pa/Pr at a specific location because Ma and Pa are at a specific location.
RE: Shear Lag Transfer Length
What I want to accomplish is to know at what location it makes sense to no longer consider the shear lag effect in the combined stress check. It sounds like there may not be a closed-form answer there, but the two suggestions you've offered seem like reasonable places to start.
RE: Shear Lag Transfer Length
RE: Shear Lag Transfer Length
Philosophically, it's probably similar to concrete where the sectional checks cease to make sense in the disturbed regions so we switch to strut and tie at the connections. We have tools for both the connections and the Bernoulli regions in steel but not much guidance with regard to the transition length.
My gut feel is that, if the connection works and the combined section checks calc out a few member depths away, you should be alright.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.